1. Field of the Invention
Disclosed herein are nanoparticle compositions of matter containing spherical nanoparticles and nanoparticles having globular, coral-like shapes and methods of making such compositions.
2. Relevant Technology
The term “nanoparticle” often refers to particles having a largest dimension of less than 100 nm. Nanoparticle research is currently an area of intense scientific interest due to a wide variety of potential applications in biomedical, optical and electronic fields.
Nanoparticles are of great scientific interest as they are, in effect, a bridge between bulk materials and atomic or molecular structures. Bulk materials typically have constant physical properties regardless of size, but at the nano-scale, size-dependent properties are often observed. Thus, properties of materials change as their size approaches the nanoscale and as the percentage of atoms at the surface of a material becomes significant. For bulk materials larger than one micrometer (or micron), the percentage of atoms at the surface is insignificant in relation to the number of atoms in the bulk of the material. The interesting and sometimes unexpected properties of nanoparticles are therefore largely due to the large surface area of the material, which dominates the contributions made by the relatively small bulk of the material.
Nanoparticles often possess unexpected optical properties as they are small enough to confine their electrons and produce quantum effects. For example gold nanoparticles appear deep-red to black in solution. Nanoparticles of yellow gold and grey silicon are red in color. Gold nanoparticles melt at much lower temperatures (˜300° C. for 2.5 nm size) than the gold slabs (1064° C.). Absorption of solar radiation is much higher in materials composed of nanoparticles than it is in thin films of continuous sheets of material. In both solar PV and solar thermal applications, controlling the size, shape, and material of the particles, it is possible to control solar absorption.
The size-dependent property changes of nanoparticles include quantum confinement in semiconductor particles, surface plasmon resonance in some metal particles, and superparamagnetism in magnetic materials. Suspensions of nanoparticles are possible since the interaction of the particle surface with the solvent is strong enough to overcome density differences, which otherwise usually result in a material either sinking or floating in a liquid.
The high surface area to volume ratio of nanoparticles provides a tremendous driving force for diffusion, especially at elevated temperatures. Sintering can take place at lower temperatures, over shorter time scales than for larger particles. In theory, this does not affect the density of the final product, though flow difficulties and the tendency of nanoparticles to agglomerate may complicate matters. Moreover, nanoparticles have been found to impart extra properties to various day-to-day products. For example, the presence of titanium dioxide nanoparticles imparts what is called the self-cleaning effect, and, the size being nano-range, the particles cannot be observed. Zinc oxide particles have been found to have superior UV blocking properties compared to its bulk substitute.
Metal, dielectric, and semiconductor nanoparticles have been formed, as well as hybrid structures (e.g., core-shell nanoparticles). Nanoparticles made of semiconducting material may also be labeled quantum dots if they are small enough (typically <10 nm) so that quantization of electronic energy levels occurs. Such nanoscale particles are typically used in biomedical applications as drug carriers or imaging agents.
There are several methods for creating nanoparticles, including both attrition and pyrolysis. In attrition, macro- or micro-scale particles can be ground in a ball mill, a planetary ball mill, or other size-reducing mechanism. The resulting particles are air classified to recover nanoparticles. In pyrolysis, a vaporous precursor (liquid or gas) is forced through an orifice at high pressure and burned. The resulting solid (a version of soot) is air classified to recover oxide particles from by-product gases. Traditional pyrolysis often results in aggregates and agglomerates rather than single primary particles. Ultrasonic nozzle spray pyrolysis (USP) is another method aimed at preventing agglomerates from forming.
A thermal plasma can also deliver the energy necessary to cause vaporization of small micrometer-size particles. The thermal plasma temperatures are in the order of 10,000 K, so that solid powder easily evaporates. Nanoparticles are formed upon cooling while exiting the plasma region. Typical thermal plasma torches used to produce nanoparticles are DC plasma jet, DC arc plasma, and radio frequency (RF) induction plasmas. In the arc plasma reactors, the energy necessary for evaporation and reaction is provided by an electric arc formed between the anode and the cathode. For example, silica sand can be vaporized with an arc plasma at atmospheric pressure. The resulting mixture of plasma gas and silica vapor can be rapidly cooled by quenching with oxygen, thus ensuring the quality of the fumed silica produced.
Scientists have taken to naming their particles after the real-world shapes that they might represent. The terms “nanospheres”, “nanoreefs”, “nanoboxes” and more have appeared in the literature. These morphologies sometimes arise spontaneously as an effect of a templating or directing agent present in the synthesis, such as miscellar emulsions or anodized alumina pores, or from the innate crystallographic growth patterns of the materials themselves. Some of these morphologies may serve a purpose, such as long carbon nanotubes used to bridge an electrical junction. Others may just serve a scientific curiosity, like the “nanostars.”
Amorphous particles usually adopt a spherical shape (due to their microstructural isotropy), whereas the shape of anisotropic microcrystalline whiskers corresponds to their particular crystal habit. At the small end of the size range, nanoparticles are often referred to as clusters. Spheres, rods, fibers, and cups are just a few of the shapes that have been grown.
Nanoparticle characterization is necessary to establish understanding and control of nanoparticle synthesis and applications. Characterization is done by using a variety of different techniques, mainly drawn from materials science. Common techniques are electron microscopy (TEM, SEM), atomic force microscopy (AFM), dynamic light scattering (DLS), x-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF), ultraviolet-visible spectroscopy, Rutherford backscattering spectrometry (RBS), dual polarisation interferometry, and nuclear magnetic resonance (NMR).
Nanoparticles can be grown into spheres through chemical reduction methods (e.g., silica), while production of spherical nanoparticles from other starting materials has traditionally been through a two-step process. In a first step, growth of nanoparticles from non-silica starting materials by chemical reduction methods produces non-spherical shapes, such as hedrons, platelets, rods, and other non-spherical shapes. While these methods provide good control for size, the resulting non-spherical shapes require further processing before they can become spherical in shape. In a second step, laser ablation is used to aggressively mill the non-spherical particles into quasi-spherical and/or spherical shapes. This process often produces unwanted “scrap” pieces and metal ions as byproduct. The spherical particles are then filtered to remove the ions and unwanted scrap.
Accordingly, there remains a need to manufacture new types of nanoparticles and nanoparticle compositions in order to provide desired properties and/or reduce harmful effects.